U.S. patent application number 12/706980 was filed with the patent office on 2011-08-18 for piezoresistive sensors for mems device having rejection of undesired motion.
This patent application is currently assigned to MICROVISION, INC.. Invention is credited to Dean R. Brown, Wyatt O. Davis, Yunfei Ma, Jason B. Tauscher.
Application Number | 20110199284 12/706980 |
Document ID | / |
Family ID | 44369300 |
Filed Date | 2011-08-18 |
United States Patent
Application |
20110199284 |
Kind Code |
A1 |
Davis; Wyatt O. ; et
al. |
August 18, 2011 |
Piezoresistive Sensors for MEMS Device Having Rejection of
Undesired Motion
Abstract
Briefly, in accordance with one or more embodiments, a
piezoresistive stress sensor comprises a plurality of
piezoresistive elements coupled in a bridge circuit disposed on,
near, or contiguous to a flexure to detect torsional flexing about
an axis of the flexure. The bridge circuit has at least two nodes
disposed along the axis of the flexure and at least two nodes
disposed off the axis of the flexure to maximize, or nearly
maximize, an output of the bridge circuit in response to the
torsional flexing of the flexure. A torsional flexing component of
the output signal of the bridge circuit is relatively increased
with respect to a component of the output signal generated by
non-torsional stress of the flexure, or a component of the output
signal generated by non-torsional stress of the flexure is reduced
with respect to the torsional flexing component of the output
signal, or combinations thereof.
Inventors: |
Davis; Wyatt O.; (Bothell,
WA) ; Ma; Yunfei; (Redmond, WA) ; Brown; Dean
R.; (Lynnwood, WA) ; Tauscher; Jason B.;
(Sammamish, WA) |
Assignee: |
MICROVISION, INC.
Redmond
WA
|
Family ID: |
44369300 |
Appl. No.: |
12/706980 |
Filed: |
February 17, 2010 |
Current U.S.
Class: |
345/31 ;
73/778 |
Current CPC
Class: |
G01L 5/0042 20130101;
G09G 3/3466 20130101; G02B 26/0833 20130101; G02B 26/105
20130101 |
Class at
Publication: |
345/31 ;
73/778 |
International
Class: |
G09G 3/00 20060101
G09G003/00; G01L 1/18 20060101 G01L001/18 |
Claims
1. A piezoresistive stress sensor, comprising: a plurality of
piezoresistive elements coupled together at a plurality of nodes in
a bridge circuit; wherein the bridge circuit is disposed on, near,
or contiguous to a flexure to detect torsional flexing about an
axis of the flexure, the bridge circuit having at least two nodes
disposed along the axis of the flexure and at least two nodes
disposed off the axis of the flexure to maximize, or nearly
maximize, an output of the bridge circuit in response to the
torsional flexing of the flexure; and a plurality of contact pads
coupled to the plurality of nodes to electrically contact with the
bridge circuit, the plurality of contact pads being sufficiently
oversized with respect to a width dimension of the plurality of
piezoresistive elements to accommodate misalignment of the contact
pads with the plurality of nodes.
2. A piezoresistive stress sensor as claimed in claim 1, wherein
the bridge circuit comprises a Wheatstone bridge.
3. A piezoresistive stress sensor as claimed in claim 1, wherein
the plurality of contact pads comprise relatively high
concentration doped silicon.
4. A piezoresistive stress sensor as claimed in claim 1, wherein
the contact pads are generally circular in shape, generally
elliptical in shape, generally polygonal in shape, or generally
non-polygonal in shape, or combinations thereof.
5. A MEMS scanning platform, comprising: a frame; a suspended body
coupled to the frame via at least one or more flexures, the
suspended body having a mirrored surface disposed thereon; and at
least one or more piezoresistive sensors disposed on, near, or
contiguous to at least one of the flexures, the at least one or
more piezoresistive sensors comprising: a plurality of
piezoresistive elements coupled together at a plurality of nodes in
a bridge circuit; and wherein a torsional flexing component of an
output signal of the bridge circuit is relatively increased with
respect to a component of the output signal generated by
non-torsional stress of the flexure, or wherein a component of the
output signal generated by non-torsional stress of the flexure is
reduced with respect to the torsional flexing component of the
output signal, or combinations thereof.
6. A MEMS scanning platform as claimed in claim 5, wherein the
bridge circuit is disposed to detect torsional flexing about an
axis of the flexure, the bridge circuit having at least two nodes
disposed along the axis of the flexure and at least two nodes
disposed off the axis of the flexure to maximize, or nearly
maximize, an output signal of the bridge circuit generated in
response to the torsional flexing of the flexure.
7. A MEMS scanning platform as claimed in claim 5, further
comprising a plurality of contact pads coupled to the plurality of
nodes to electrically contact with the bridge circuit, the
plurality of contact pads being sufficiently oversized with a
respect to a width dimension of the plurality piezoresistive
elements to accommodate misalignment of the contact pads with the
plurality of nodes.
8. A MEMS scanning platform as claimed in claim 7, wherein the
plurality of contact pads comprise high concentration doped
silicon.
9. A MEMS scanning platform as claimed in claim 7, wherein the
contact pads are generally circular in shape, generally elliptical
in shape, generally polygonal in shape, or generally non-polygonal
in shape, or combinations thereof.
10. A MEMS scanning platform as claimed in claim 5, wherein the
bridge circuit comprises a Wheatstone bridge.
11. A MEMS scanning platform as claimed in claim 5, wherein: a
first piezoresistive sensor is disposed on, near, or contiguous to
a first flexure at a first position along the first flexure, and
second piezoresistive sensor is disposed on, near, or contiguous to
a second flexure at a second position along the second flexure
symmetrical or nearly symmetrical to the first position, the first
and second piezoresistive sensors having a mutual bias voltage and
a combined output such that the combined output has a torsional
flexure component that is approximately twice the torsional flexing
component of the respective first or second piezoresistive sensors,
and such that non-torsional components of the respective first and
second piezoresistive sensors cancel out, or nearly cancel out, the
combined output.
12. A MEMS scanning platform as claimed in claim 5, wherein: the at
least one or more piezoresistive elements comprises a first pair of
piezoresistive elements disposed on, near, or contiguous to on,
near, or contiguous to a first flexure at a first position along
the first flexure, and second pair piezoresistive elements disposed
on, near, or contiguous to a second flexure at a second position
along the second flexure symmetrical or nearly symmetrical to the
first position, the first pair and the second pair of
piezoresistive elements being coupled to form a single
piezoresistive sensor in a bridge circuit having an output signal
representative of stress of the first flexure and the second
flexure such that non-torsional components of the respective first
and second pairs of piezoresistive elements cancel out, or nearly
cancel out, the output signal.
13. A scanned beam display, comprising: at least one or more light
sources to generate a light beam; and a MEMS scanning platform to
scan the beam of light in a pattern, the MEMS scanning platform
comprising: a frame; a suspended body coupled to the frame via at
least one or more flexures, the suspended body having a mirrored
surface disposed thereon; and at least one or more piezoresistive
sensors disposed on, near, or contiguous to at least one of the
flexures, the at least one or more piezoresistive sensors
comprising a plurality of piezoresistive elements coupled together
at a plurality of nodes in a bridge circuit; and wherein a
torsional flexing component of an output signal of the bridge
circuit is relatively increased with respect to a component of the
output signal generated by non-torsional stress of the flexure, or
wherein a component of the output signal generated by non-torsional
stress of the flexure is reduced with respect to the torsional
flexing component of the output signal, or combinations
thereof.
14. A scanned beam display as claimed in claim 13, wherein the
bridge circuit is disposed to detect torsional flexing about an
axis of the flexure, the bridge circuit having at least two nodes
disposed along the axis of the flexure and at least two nodes
disposed off the axis of the flexure to maximize, or nearly
maximize, an output signal of the bridge circuit generated in
response to the torsional flexing of the flexure.
15. A scanned beam display as claimed in claim 13, the bridge
circuit further comprising a plurality of contact pads coupled to
the plurality of nodes to electrically contact with the bridge
circuit, wherein the contact pads are generally circular in shape,
generally elliptical in shape, generally polygonal in shape, or
generally non-polygonal in shape, or combinations thereof.
16. A scanned beam display as claimed in claim 15, wherein the
plurality of contact pads of the bridge circuit comprise high
concentration doped silicon.
17. A scanned beam display as claimed in claim 15, wherein the
plurality of contact pads of the bridge circuit are sufficiently
oversized with respect to a width dimension of the plurality of
piezoresistive elements to accommodate misalignment of the contact
pads with the plurality of nodes.
18. A scanned beam display as claimed in claim 13, wherein the
bridge circuit comprises a Wheatstone bridge.
19. A scanned beam display as claimed in claim 13, wherein: a first
piezoresistive sensor is disposed on, near, or contiguous to a
first flexure at a first position along the first flexure, and
second piezoresistive sensor is disposed on, near, or contiguous to
a second flexure at a second position along the second flexure
symmetrical or nearly symmetrical to the first position, the first
and second piezoresistive sensors having a mutual bias voltage and
a combined output such that the combined output has a torsional
flexure component that is approximately twice the torsional flexing
component of the respective first or second piezoresistive sensors,
and such that non-torsional components of the respective first and
second piezoresistive sensors cancel out, or nearly cancel out, the
combined output.
20. A scanned beam display as claimed in claim 13, the at least one
or more piezoresistive elements comprises a first pair of
piezoresistive elements disposed on, near, or contiguous to a first
flexure at a first position along the first flexure, and second
pair piezoresistive elements disposed on, near, or contiguous to a
second flexure at a second position along the second flexure
symmetrical or nearly symmetrical to the first position, the first
pair and the second pair of piezoresistive elements being coupled
to form a single piezoresistive sensor in a bridge circuit having
an output signal representative of stress of the first flexure and
the second flexure such that non-torsional components of the
respective first and second pairs of piezoresistive elements cancel
out, or nearly cancel out, the output signal.
Description
BACKGROUND
[0001] Piezoresistive (PZR) stress sensors can be used for position
sensing for microelectromechanical system (MEMS) scanning mirrors
in applications such as scanned beam display systems. The amount of
mechanical strain detected by the sensor is related to the position
of the scanning mirror. Thus, a strain signal obtained by the
sensor may be used as a feedback input in the scanning control
system to provide correction to the drive signal to facilitate
control of the movement of the scanning mirror. Ideally, the strain
sensor is capable of producing a signal in response only to a
certain kind of motion of the scanning mirror, for example
torsional deformation of the flexures supporting the scanning
mirror. In practice however, such sensors may be sensitive to other
types of motion which result from deviation from the ideal case
caused by material fabrication or process variation. For example
variations in certain properties of the sensor can cause it to
generate a feedback signal having a component due to bending
deformation of the flexures in addition to the torsional response.
Consequently, the detection of undesired deformations by the sensor
may cause the scanning control system to apply a less than ideal
and/or an incorrect control signal since the feedback signal from
the sensor may contain components due to motion other than the type
of motion being controlled.
DESCRIPTION OF THE DRAWING FIGURES
[0002] Claimed subject matter is particularly pointed out and
distinctly claimed in the concluding portion of the specification.
However, such subject matter may be understood by reference to the
following detailed description when read with the accompanying
drawings in which:
[0003] FIG. 1 is a block diagram of a control system for a
microelectromechanical system (MEMS) platform in accordance with
one or more embodiments;
[0004] FIG. 2 is a diagram of feedback electronics for
microelectromechanical system (MEMS) platform control system in
accordance with one or more embodiments;
[0005] FIG. 3A and FIG. 3B are a top view diagrams of a
microelectromechanical system (MEMS) scanning platform having
piezoresistive position sensors in accordance with one or more
embodiments;
[0006] FIG. 4 is a diagram illustrating detail of piezoresistive
position sensors in a bridge arrangement in accordance with one or
more embodiments;
[0007] FIG. 5 is a diagram of a layout for the PZR elements of a
PZR sensor and respective electrical contacts illustrating layer
misalignments in accordance with one or more embodiments;
[0008] FIG. 6 is a illustration of a layout for the PZR elements of
a PZR sensor and respective electrical contacts to reduce
sensitivity to layer misalignments in accordance with one or more
embodiments;
[0009] FIG. 7 is a diagram of a dual bridge piezoresistive sensor
arrangement in accordance with one or more embodiments;
[0010] FIG. 8 is a diagram of a split bridge piezoresistive sensor
arrangement in accordance with one or more embodiments;
[0011] FIG. 9 is a diagram of a scanned beam display in accordance
with one or more embodiments;
[0012] FIG. 10 is a diagram of an information handling system
utilizing a scanned beam display in accordance with one or more
embodiments.
[0013] It will be appreciated that for simplicity and/or clarity of
illustration, elements illustrated in the figures have not
necessarily been drawn to scale. For example, the dimensions of
some of the elements may be exaggerated relative to other elements
for clarity. Further, if considered appropriate, reference numerals
have been repeated among the figures to indicate corresponding
and/or analogous elements.
DETAILED DESCRIPTION
[0014] In the following detailed description, numerous specific
details are set forth to provide a thorough understanding of
claimed subject matter. However, it will be understood by those
skilled in the art that claimed subject matter may be practiced
without these specific details. In other instances, well-known
methods, procedures, components and/or circuits have not been
described in detail.
[0015] In the following description and/or claims, the terms
coupled and/or connected, along with their derivatives, may be
used. In particular embodiments, connected may be used to indicate
that two or more elements are in direct physical and/or electrical
contact with each other. Coupled may mean that two or more elements
are in direct physical and/or electrical contact. However, coupled
may also mean that two or more elements may not be in direct
contact with each other, but yet may still cooperate and/or
interact with each other. For example, "coupled" may mean that two
or more elements do not contact each other but are indirectly
joined together via another element or intermediate elements.
Finally, the terms "on," "overlying," and "over" may be used in the
following description and claims. "On," "overlying," and "over" may
be used to indicate that two or more elements are in direct
physical contact with each other. However, "over" may also mean
that two or more elements are not in direct contact with each
other. For example, "over" may mean that one element is above
another element but not contact each other and may have another
element or elements in between the two elements. Furthermore, the
term "and/or" may mean "and", it may mean "or", it may mean
"exclusive-or", it may mean "one", it may mean "some, but not all",
it may mean "neither", and/or it may mean "both", although the
scope of claimed subject matter is not limited in this respect. In
the following description and/or claims, the terms "comprise" and
"include," along with their derivatives, may be used and are
intended as synonyms for each other.
[0016] Referring now to FIG. 1, a block diagram of a control system
for a microelectromechanical system (MEMS) platform in accordance
with one or more embodiments will be discussed. As shown in FIG. 1,
control system 100 may comprise controller 122 providing a drive
signal 112 to a microelectromechanical (MEMS) platform 114. In one
or more embodiments, MEMS platform 114 may be part of a scanned
beam display such as shown in and described with respect to FIG. 9,
below. In general, MEMS platform 114 may comprise any type of MEMS
device that operates electromechanically in response to a drive
signal 112, and the scope of the claimed subject matter is not
limited in this respect. Control system 100 may further include
feedback electronics 110 that include one or more sensors to detect
the motion of MEMS platform 114 to provide a control signal 116 as
a feedback signal to controller 122 to facilitate the drive signal
112 applied to MEMS platform 114. As discussed further herein, such
a sensor may comprise a piezorestive (PZR) sensor that is capable
of transducing mechanical movement of at least a portion of MEMS
platform 114 into an electrical signal to generate control signal
116. Further details of an example embodiment of feedback
electronics 110 including a PZR sensor for control system 110 are
shown in and described with respect to FIG. 2, below.
[0017] Referring now FIG. 2, a diagram of feedback electronics for
microelectromechanical system (MEMS) platform control system in
accordance with one or more embodiments will be discussed. In one
or more embodiments of feedback electronics 110, an amplification
stage 222 may be used to create a control signal 116, labeled as
V.sub.CONTROL in FIG. 2, from an output signal V.sub.OUT taken at
node 218 and node 220 of a PZR sensor 210. In one or more
embodiments, PZR sensor comprises PZR elements 212 arranged in a
bridge circuit configuration as shown in FIG. 2, although the scope
of the claimed subject matter is not limited in this respect.
Amplification stage 222 may be an instrumentation amplifier, for
example an operational amplifier as shown in FIG. 2, and/or may
comprise one or more amplification stages capable of providing gain
or attenuation of the output signal. Although one amplification
stage 222 is shown in FIG. 2, multiple amplification stages may be
utilized in one or more alternative embodiments. The control signal
116 V.sub.CONTROL provided by amplification stage 222 may be used
in feedback control systems, such as control system 100 of FIG. 1,
that generate horizontal drive (HDrive) and/or vertical drive
(VDrive) signals to control the amplitude, frequency, phase, and/or
other aspects of the motion of MEMS platform 114 of FIG. 1, and
that cause the PZR sensor 210 to respond with an output signal
V.sub.OUT in response to the motion. For example, the motion may be
the rotation a suspended body containing a scanning mirror, and/or
the rotations of a frame of MEMS platform 114 as shown in and
described with respect to FIG. 3, below. In some embodiments, MEMS
platform 114 may comprise a MEMS die fabricated from silicon,
wherein amplification stage or stages 222 may be implemented with
discrete electronics connected to the MEMS die, or amplification
stages 222 alternatively may be fabricated integrally to the MEMS
die using semiconductor integrated circuit technology. Control
system 100 may be utilized to control the biaxial scan trajectory
of MEMS platform 114. In one or more embodiments as discussed in
further detail herein, to optimize the performance of control
system 100, the output signal V.sub.OUT of PZR sensor ideally 210
should correspond only to one type of motion, for example only to
the rotations of the body 310 or frame 328 of MEMS platform 114 as
shown in and described with respect to FIG. 3, below. If the output
signal V.sub.OUT of PZR sensor 210 contains contributions from
other types of motion such as translations or rotations of the body
310 or frame 328 of MEMS platform 114 that put the corresponding
flexures into bending instead of torsion, the operation of control
system 100 may be less than optimal or more complex to account for
such additional motions. If the additional motions detected by PZR
sensor 210 become too severe, such sensitivity to ancillary motion
may degrade the image quality in a biaxial scan display system such
as scanned beam display 900 of FIG. 9, below. In one or more
embodiments, the response of PZR sensor 210 to ancillary motions
may be reduced or effectively eliminated as discussed in further
detail herein. Structural details of an example MEMS platform 114
that incorporates a PZR sensor 210 are shown in and described with
respect to FIG. 3, below.
[0018] Referring now FIG. 3A and FIG. 3B, top view diagrams of a
microelectromechanical system (MEMS) platform having piezoresistive
position sensors in accordance with one or more embodiments will be
discussed. As shown in FIG. 3A and FIG. 3B, the MEMS platform 114
of FIG. 1 may comprise a microelectromechanical system based
scanner comprising a suspended body 310 having scanning mirror 320
formed thereon. Suspended body 310 is suspended by flexure 312
and/or flexure 314 which in turn are coupled to a frame 328. During
operation of MEMS platform 114, suspended body 310 may be actuated
to rotate about axis 326 via torsional movement of the flexures via
a motor circuit (not shown). Frame 328 and flexures 312 and 314 may
be made of silicon or the like which provides sufficient support
and flexing characteristics to allow such movement of suspended
body 310 when driven via the motor circuit. In one or more
embodiments, flexures 312 and 314 may be fabricated from a
different material than the material from which frame fabricated.
In such embodiments, a silicon piezoresistive (PZR) sensor 210
fabricated from silicon may be arranged to react to torsional
response of a flexure 312 or 314 that is attached to frame 328.
However, this is merely an example arrangement of frame 328 and
flexures 312 and 314, and the scope of the claimed subject matter
is not limited in this respect. When MEMS platform 114 is used in
scanned beam display 900 of FIG. 9, below, the rotational movement
of suspended body 310 about axis 326 is used for scanning of beam
912 along a first axis in a projected image. Frame 328 may be
supported by flexure 316 and flexure 318 along axis 330. Scanning
of beam 912 about a second axis that is orthogonal to the first
axis may be accomplished via rotational movement of suspended body
310 long with frame 328 about axis 330 via further actuation of the
motor circuit. The combination of movement of suspended body 310
about axis 326 and axis 330 allows a two-dimensional image to be
generated by scanned beam display 100 as discussed herein.
[0019] During scanning movement of suspended body 310, a feedback
signal may be obtained that is indicative of the amount of
rotational movement of suspended body 310 about axis 326. The
feedback signal may then be utilized by controller 122 of FIG. 1 to
provide any needed correction in the drive signal 112 applied to
the motor circuit driving MEMS platform 114 as control signal 116.
Such a feedback signal may be obtained by a piezoresistive (PZR)
sensor 210 disposed on or near or contiguous to at least one of the
flexures such as flexure 314 and/or flexure 318. In the embodiment
shown in FIG. 3A, a PZR sensor 210 is shown disposed on flexure
314, and in the embodiment shown in FIG. 3B, a PZR sensor 210 is
shown disposed on frame 328 rather than being disposed on either
flexure 312 or 314. In general, one or more PZR sensors 210 may be
located at any one or more various locations including on, near, or
contiguous to one or more flexures such as flexure 312, flexure
314, flexure 316 and/or flexure 318, and/or on, near, or contiguous
to frame 328, and the scope of the claimed subject matter is not
limited in these respects. PZR sensor 210 may comprise one or more
PZR elements as shown and described in further detail with respect
to FIG. 4, below. A PZR element may refer to an element having a
resistance that is a function of the amount of mechanical stress
within the element. The coefficient of resistivity dependence on a
stress .sigma. may be denoted by the symbol .pi.. Thus, a PZR
element may have a resistance expressed as:
R(.sigma.)=R.sub.0+.pi..sigma.
wherein the resistance of the PZR element is equal to the
resistance of the PZR element with no stress plus the coefficient
of resistivity times the amount of added stress. In one or more
embodiments, PZR sensor 210 may comprise multiple PZR elements in a
bridge circuit arrangement to provide a differential output as the
feedback signal, for example as shown in and described with respect
to FIG. 4, below. Thus, an input signal V.sub.IN applied to PZR
sensor 210 via terminals 322 may result in an output signal
V.sub.OUT at terminals 324 proportional to the change in resistance
in the PZR elements of the sensor 210, in response to the amount of
flex of flexure 314 which is representative of the rotational
position of suspended body 310. In one or more embodiments, the
input signal V.sub.IN is a direct current (dc) signal that results
in an output signal V.sub.OUT that varies with amount of torsional
flexing of flexure 314 as the resistance of the PZR elements of PZR
sensor 210 changes in response to stress in flexure 314. Further
details of PZR sensor 210 are shown in and described with respect
to FIG. 4, below.
[0020] Referring now to FIG. 4, a diagram illustrating detail of
piezoresistive position sensors in a bridge arrangement in
accordance with one or more embodiments will be discussed. FIG. 4
shows one example arrangement of PZR sensor 210 as shown in and
described with respect to FIG. 2 and FIG. 3, above. In one or more
embodiments, PZR sensor 210 comprises multiple PZR elements such as
PZR element 412 having a resistance R1, PZR element 414 having a
resistance R2, PZR element 416 having a resistance R4, and PZR
element 418 having a resistance R3, in a bridge circuit
arrangement, also referred to as a Wheatstone bridge. In such a
bridge circuit, a given PZR element is coupled to an adjacent PZR
element via one of four nodes 424. In one or more particular
embodiments, PZR sensor 210 may be generally aligned with axis 326
such that two of the nodes 424 are generally aligned on axis 326
and the other two of the nodes 424 are disposed off of axis 326
resulting in bridge 210 being generally oriented in a diamond
shaped arrangement with respect to flexure axis 326. In general in
one or more embodiments, such orienting of the PZR elements at an
approximately 45 degree angle with respect to torsion axis 326 of
flexure 314 may maximize, or nearly maximize, the output signal of
PZR sensor 210 in response to torsional stress of flexure 314.
Frame 328 of scanning platform 114 of FIG. 3 may be fabricated such
that the crystal axis 410 (represented as <110>) shown in
FIG. 4 is generally aligned parallel to flexure axis 326, although
the scope of the claimed subject matter is not limited in this
respect.
[0021] For the bridge circuit arrangement of PZR sensor 210, let i
be an index representing the number of PZR elements in the bridge
circuit such that i=1, . . . , 4. The resistance of each PZR
element can be decomposed as the sum of a nominal value R.sub.0 and
a variation from the nominal value .DELTA.R.sub.i:
R.sub.i=R.sub.0+.DELTA.R.sub.i
The output voltage V.sub.OUT can be expressed as:
V OUT = V IN ( R 0 + .DELTA. R 2 2 R 0 + .DELTA. R 1 + .DELTA. R 2
- R 0 + .DELTA. R 4 2 R 0 + .DELTA. R 3 + .DELTA. R 4 ) ,
##EQU00001##
which has a simplified expression when all the resistance
variations .DELTA.Ri are equal to .DELTA.R:
V OUT = - V IN ( .DELTA. R R 0 ) . ##EQU00002##
In general the resistance variations will be unequal. Situations
that may cause resistance variation inequality in the values of the
PZR elements are discussed with respect to FIG. 5 and FIG. 6,
below. In one or more embodiments, the resistance variations
.DELTA.Ri may be due to differences in the nominal, unstressed
resistance of the PZR elements, and differences in their
piezoresistance changes due to stress. The resistance variations of
the PZR elements can be decomposed according to:
.DELTA.R.sub.i=.DELTA.R.sub.i0+.pi..sub.i .sigma..sub.i
The first term on the right-hand side of the equation is due to
variations in the nominal resistance. The second term is the
product of the piezoresistance coefficient, or .pi.-coefficient,
and the stress .sigma. in the element.
[0022] When the PZR elements are implanted in a single crystal
semiconductor material such as silicon, each PZR element in the
bridge circuit of PZR sensor 210 has a sense of direction parallel
to the net direction current flow 426 between regions in a given
PZR element that have higher conductivity relative to the
piezoresistive portion, having an angle .alpha. relative to the
crystal axis 410. Let i be an index representing the number of PZR
elements in the bridge circuit such that i=1, . . . , 4 then the
average it coefficient .pi..sub.i for each PZR element is an even
function of .alpha.i such that
.pi..sub.i=.pi.(.alpha.i)=.pi.(-.alpha.i). With matched nominal
resistances .DELTA.R.sub.i0, the resistance variation .DELTA.Ri for
PZR sensor 210 may be decomposed as follows:
.DELTA.Ri=(.PI.0+.DELTA..pi.i)(S0+.DELTA..sigma.i)
The ideal case is a balanced situation that is achieved when:
.alpha.1=-.alpha.2=-.alpha.3=.alpha.4
which corresponds to the case of balanced nominal resistance and
piezoresistance examined above, providing a torsion sensor with
bending immunity. The bending immunity is preserved even in the
presence of a gradient in the bending stress field such that
.sigma.1=.sigma.3=S1, .sigma.2=.sigma.4=S2.
[0023] A longitudinal stress gradient may appear for pure torsional
deformation of flexure 314 of FIG. 3 because the cross section of
flexure 314 changes along the portion of the axis of flexure 314
containing the PZR sensor 210. A longitudinal stress gradient can
also occur due to out-of-plane bending of flexure 314, however for
that case .sigma.1=.sigma.3=S1, .sigma.2=.sigma.4=S2, that is there
are no sign reversals of the stress over the PZR sensor 210 and so
.DELTA.R1=.DELTA.R3, .DELTA.R2=.DELTA.R4. In that case the
differential output signal V.sub.OUT=0.
[0024] If the patterning process for forming the PZR elements
creates an alignment error, as discussed with respect to FIG. 5 and
FIG. 6, below, between the intended resistor orientations and the
crystal axis, such as .alpha.1=.alpha.4=A1, .alpha.2=.alpha.3=A2,
A1.noteq.A2, then the .pi. coefficients will be such that
.pi.1=.pi.4=.PI.1, .pi.2=.pi.3=.PI.2, .PI.1.noteq..PI.2. For
torsion, this imbalance will create nonlinear distortion of the
output signal V.sub.OUT, made worse when there are stress
gradients. Likewise, for bending, the imbalance causes the bridge
PZR sensor 210 to output a differential signal that would be
indistinguishable from a torsion signal, and that is nonlinearly
distorted due to nonlinear dependence on the stress.
[0025] The alignment inaccuracy of the highly conductive layers
that make contact to the PZR elements may create imbalance in the
PZR elements that creates a dc differential output when there is no
stress in the torsion member. In addition, this misalignment may
cause deviations from what is intended for the directions of net
current flow relative to the crystal axes 410 for the PZR elements.
That is, the angles .alpha.i may vary. In this situation, torsional
deformation of the flexure 314 results in nonlinear distortion of
the bridge output V.sub.OUT of PZR sensor 210, such distortion is
made worse when there are stress gradients. Furthermore, the PZR
bridge is nonlinearly responsive to bending stress, such
nonlinearity may be more severe when there are stress gradients
across the bridge. The misalignment of the angles .alpha.i
describing the sense of net current flow in each PZR element
therefore degrades the bending immunity of the sensor.
[0026] To address alignment inaccuracy, contact pads 422 for PZR
sensor 210 may be fabricated from relatively high concentration
doped silicon at nodes 424 of PZR sensor 210. In one or more
embodiments, contacts 422 are generally rounded in shape and
relatively large with respect to nodes 424 such that some amount of
process variation misalignment in the doping patterns from which
contacts 422 are formed may be tolerated. However, it should be
noted that this is merely one example of the possible shape and/or
size of contact pads 422 to accommodate process variation wherein
other sizes and/or shapes likewise may be utilized, and the scope
of the claimed subject matter is not limited in this respect. For
example, such other geometries, sizes, and/or shapes of contact
pads 422 may include circular shapes, ellipsoid shapes, polygonal
shapes, and/or non-polygonal shapes, and the scope of the claimed
subject matter is not limited in these respects. Further details of
how alignment inaccuracy in the layers of PZR sensor 210 may be
accommodated are shown in and described with respect to FIG. 5 and
FIG. 6, below.
[0027] Referring now to FIG. 5, a layout for the PZR elements of a
PZR sensor and respective electrical contacts illustrating layer
misalignments in accordance with one or more embodiments will be
discussed. In one or more embodiments, variations in the nominal
resistance of each of the PZR elements in the PZR sensor 210 may be
due to the misalignment of layers during fabrication of the bridge.
For a PZR sensor 210 in a bridge circuit formed from a
piezoresistive doping implant layer and a contact layer comprising
contact pads 422 used to make electrical connections to the PZR
sensor 210 but with insignificant piezoresistivity, the alignment
between the two layers may cause resistance asymmetry. The
arrangement 510 on the lefts shows ideal alignments and therefore
equal or sufficiently equal resistance on all PZR elements forming
the legs of the bridge. In arrangement 510, contact pads 422 are
generally aligned with the center 514 of the PZR sensor 210 bridge.
The arrangement 512 on the right shows a misalignment of contact
pads 422 with the center 514 of the PZR sensor 210 bridge, which
therefore may result in an unequal resistance in the PZR elements
forming the legs of the bridge. Such layer misalignments may result
in rotations of the angles .alpha..sub.i of the PZR elements with
respect to crystal axis 410. For a PZR sensor 210 bridge positioned
on one of the flexures of MEMS platform 114, a torsional
deformation of a flexure with matched nominal resistances and
matched piezoresistances may be:
.DELTA. R i 0 = 0 , i = 1 , 4 ##EQU00003## .pi. i = .pi. 0 , i = 1
, 4 ##EQU00003.2## .sigma. 1 = - .sigma. 2 = - .sigma. 3 = .sigma.
4 = S T . .DELTA. R 1 = - .DELTA. R 2 = - .DELTA. R 3 = .DELTA. R 4
= .pi. 0 S 0 ##EQU00003.3## V OUT = - V IN ( S T .pi. 0 R 0 ) .
##EQU00003.4##
For the case of bending of a flexure, for example bending of
flexure 318 by rotation of body 328 about axis 326, the stress does
not reverse sign over the area of the PZR sensor 210 bridge.
Furthermore, if there is no significant gradient in the stress over
the dimensions of the PZR bridge, then:
.sigma.1=.sigma.2=.sigma.3=.sigma.4=S.sub.B
V.sub.OUT=0.
The PZR sensor 210 bridge may be immune to the bending stress and
may serve purely as a torsion sensor. If there are differences in
nominal PZR element resistances, for example due to a layer
misalignment as shown at arrangement 512 of FIG. 5, then the
torsion response of the PZR sensor 210 contains a dc offset voltage
as well as a dynamic component proportional to the dynamic stress.
The torsion signal may be determined by ignoring the dc signal
level. In one or more embodiments, the dc signal component may be
removed from the output signal V.sub.OUT of PZR sensor 210 via ac
coupling of PZR sensor to amplification stage 222, for example
using a coupling capacitor. Differences in the nominal PZR
resistances of the PZR elements may also make the PZR sensor 210
bridge sensitive to bending stress. Similar to the torsion case
with unbalanced differences in the nominal PZR element resistances,
there may be a dc component and a dynamic signal component
depending on the bending stress at the PZR sensor 210.
[0028] Referring now to FIG. 6, a layout for the PZR elements of a
PZR sensor and respective electrical contacts to reduce sensitivity
to layer misalignments in accordance with one or more embodiments
will be discussed. In the embodiment shown in FIG. 6, the contact
pads 422 are oversized and made roughly circular in shape or have
other geometries or shapes as discussed, above. In general, such an
oversized contact pad 422 may be sufficiently oversized to
accommodate misalignment of contact pads 422 with the nodes of PZR
sensor 210 bridge. In one example embodiment, oversized may mean
that contact pads 422 are at least 30% larger in size than a width
of the legs or branches of PZR sensor 210. In another example
embodiment, oversized may mean that contact pads 422 are at least
50% larger in size than a width of the legs or branches of PZR
sensor 210. However, these are merely examples of how contact pads
422 may be oversized, and the scope of the claimed subject matter
is not limited in these respects. Arrangement 610 on the left shows
the contact pads 422 being generally aligned with the center 514 of
PZR sensor 210 bridge. As shown in arrangement 612 on the right, in
spite of layer misalignment wherein the contact pads 422 are
misaligned with the center 514 of PZR sensor 210 bridge, the
nominal resistances of the PZR elements are not significantly
changed, and the resistance imbalance due to misalignment is
reduced, or otherwise effectively eliminated. As a result, PZR
sensor 210 bridge has increased stress immunity as a result of the
oversized and generally circularly shaped contact pads 422.
Furthermore, the arrangement of contact pads 210 and PZR elements
tends to eliminate the dependence of a, on the layer misalignment.
However, this is merely one example of how layer misalignment may
be accommodated in one or more embodiments of PZR sensor 210, and
the scope of the claimed subject matter is not limited in this
respect.
[0029] Referring now to FIG. 7, a diagram of a dual bridge
piezoresistive sensor arrangement in accordance with one or more
embodiments will be discussed. FIG. 5 shows a portion of the
scanning platform 114 depicted in FIG. 3 in which a first PZR
sensor 210 is disposed on one flexure 312 and a second PZR sensor
210 is disposed another flexure 314 in a dual PZR sensor 210
arrangement. In such a dual PZR sensor 210 arrangement, the torsion
strain output may be approximately doubled which mitigates the
effect of bending strain in the output signal of PZR sensor 210. As
shown in FIG. 7, two identical PZR sensors 210 may be disposed on
corresponding symmetric locations of flexure 312 and flexure 314.
The two PZR sensors 210 are coupled in parallel to the same bias
circuit at terminals VB+ and VB- to ensure same biasing voltage is
applied to both PZR sensors 210. For torsional strain the outputs
of the two PZR sensors 210, taken at terminals VO1+ and VO1- for
one sensor and terminals VO2+ and VO2- of the other sensor, will
have the same magnitude and phase, whereas for bending strain the
outputs of the two PZR sensors 210 are out-of-phase with same
magnitude. By adding the output signals of the two PZR sensors 210
together, the total output for torsion strain is doubled and the
total response to bending strain will be cancelled out, or
effectively canceled out. It should be noted that various other
arrangements of one or more PZR sensors 210 similarly may be
provided to enhance the output signal due to torsion strain with
respect to bending strain, and the scope of the claimed subject
matter is not limited in this respect. An alternative example
arrangement of a PZR sensor 210 is shown in and described with
respect to FIG. 8, below.
[0030] Referring now to FIG. 8, a diagram of a split bridge
piezoresistive sensor arrangement in accordance with one or more
embodiments will be discussed. The arrangement PZR sensor 210 of
FIG. 8 is substantially similar to the arrangement of two PZR
sensors 210 as shown in and described with respect to FIG. 7,
above, except that a single PZR sensor 210 may be used in a split
sensor arrangement. Two PZR elements 418 and 420 comprising one
half of PZR sensor 210 are disposed on flexure 312, and the other
two PZR elements 414 and 416 comprising the other half of PZR
sensor 210 are disposed on flexure 314. The combination of PZR
elements 414, 416, 418, and 420 forms a Wheatstone bridge circuit
wherein a bias voltage is applied between nodes VB+ and VB-, and
the output signal of PZR sensor 210 is obtained between nodes VO+
and VO-. Normally, the Wheatstone bridge circuit should have zero
output for bending strain if all four arms are subjected to uniform
bending stress state. However, in practice, the small bending
stress gradient across each bridge arm may cause noise signal due
to the unbalanced resistance change. As shown in FIG. 8, by
splitting the Wheatstone bridge circuit of PZR sensor 210 onto
corresponding symmetric locations of the respective flexures 312
and 314, all PZR element arms may be subjected to an identical
bending stress state since the two halves of PZR sensor 210 are
located in the same corresponding positions of the two respective
flexures 312 and 314. As a result, the output signal component of
PZR sensor 210 due bending stress may be reduced or mitigated, or
effectively canceled, with respect to the output signal component
due to torsion because the component signals due to bending stress
on the two halves of PZR sensor 210 are equal in magnitude but
opposite in phase on the respective flexures 312 and 314. It should
be noted that this is merely one example of how the output signal
of PZR sensor 210 due to bending may be reduced with respect to the
output signal of PZR sensor 210 due to torsion wherein various
other approaches likewise may be implemented, and the scope of the
claimed subject matter is not limited in this respect. In general,
in one or more embodiments, the torsion output signal may be
increased with respect to the bending output signal, or in one or
more embodiments the bending output signal may be reduced with
respect to the torsion output signal, or combinations thereof. An
example of how a scanned beam display may incorporate a MEMS
platform 114 having one or more PZR sensors 210 in accordance with
one or more embodiments is shown in and described with respect to
FIG. 9, below.
[0031] Referring now to FIG. 9, a diagram of a scanned beam display
in accordance with one or more embodiments will be discussed.
Scanned beam display 900 of FIG. 9 may include one or more sensors
to detect the position of scanning mirror 320 of MEMS platform 114
as discussed in detail, above. Although FIG. 9 illustrates a
scanned beam display for purposes of discussion, it should be noted
that other types of devices similarly may incorporate a MEMS
platform 114 having one or more PZR sensors 210, including but not
limited to uniaxial scanning mirrors, pressure sensors, inertial
sensors, and so on, and the scope of the claimed subject matter is
not limited in this respect.
[0032] As shown in FIG. 9, scanned beam display 900 comprises a
light source 910, which may be a laser light source or the like,
capable of emitting a beam 912 which may comprise a laser beam. In
general, scanned beam display 900 may also be referred to as a
projector. The beam 912 is incident on a MEMS platform 114 and
reflects off of scanning mirror 320 to generate a controlled output
beam 924. In one or more alternative embodiments, MEMS platform 114
may comprise one or more scanning mirror elements 320 in a variety
of physical configurations. A horizontal drive circuit 918 and/or a
vertical drive circuit 920 modulate the direction in which scanning
mirror 320 is deflected to cause output beam 924 to generate a
biaxial scan 926, thereby creating a displayed image, for example
on a display screen and/or image plane 928. Controller 122 controls
horizontal drive circuit 918 and vertical drive circuit 920 by
converting pixel information of the input image into laser
modulation synchronous to the motion of MEMS platform 114 to write
the image information as a displayed image based upon the position
of the output beam 924 in raster pattern 926 and the corresponding
intensity and/or color information at the corresponding pixel in
the image. Controller 122 may also control other various functions
of scanned beam display 900.
[0033] In one or more embodiments, a horizontal axis may refer to
the horizontal direction of biaxial scan 926 and the vertical axis
may refer to the vertical direction of biaxial scan 926. Scanning
mirror 320 may sweep the output beam 924 horizontally at a
relatively higher frequency and also vertically at a relatively
lower frequency and with a constant velocity over a portion of the
scan. The result is a scanned trajectory of laser beam 924 to
result in biaxial scan 926. The fast and slow axes may also be
interchanged such that the fast scan is in the vertical direction
and the slow scan is in the horizontal direction. However, the
scope of the claimed subject matter is not limited in these
respects.
[0034] Referring now to FIG. 10, a diagram of an information
handling system utilizing a scanned beam display in accordance with
one or more embodiments will be discussed. Information handling
system 1000 of FIG. 10 may tangibly embody scanned beam display 900
as shown in and described with respect to FIG. 9, above. Although
information handling system 1000 represents one example of several
types of computing platforms, including cell phones, personal
digital assistants (PDAs), netbooks, notebooks, internet browsing
devices, music and/or video players, and so on, information
handling system 1000 may include more or fewer elements and/or
different arrangements of the elements than shown in FIG. 10, and
the scope of the claimed subject matter is not limited in these
respects.
[0035] Information handling system 1000 may comprise one or more
processors such as processor 1010 and/or processor 1012, which may
comprise one or more processing cores. One or more of processor
1010 and/or processor 1012 may couple to one or more memories 1016
and/or 1018 via memory bridge 1014, which may be disposed external
to processors 1010 and/or 1012, or alternatively at least partially
disposed within one or more of processors 1010 and/or 1012. Memory
1016 and/or memory 1018 may comprise various types of
semiconductor-based memory, for example volatile type memory and/or
non-volatile type memory. Memory bridge 1014 may couple to a
video/graphics system 1020 to drive a display device, which may
comprise MEMS projector module 1036, coupled to information
handling system 1000. MEMS projector module 1036 may comprise
scanned beam display 1000 as shown in and described with respect to
the various figures herein. In one or more embodiments,
video/graphics system 1020 may couple to one or more of processors
1010 and/or 1012 and may be disposed on the same core as the
processor 1010 and/or 1012, although the scope of the claimed
subject matter is not limited in this respect.
[0036] Information handling system 1000 may further comprise
input/output (I/O) bridge 1022 to couple to various types of I/O
systems. I/O system 1024 may comprise, for example, a universal
serial bus (USB) type system, an IEEE 1394 type system, or the
like, to couple one or more peripheral devices to information
handling system 1000. Bus system 1026 may comprise one or more bus
systems such as a peripheral component interconnect (PCI) express
type bus or the like, to connect one or more peripheral devices to
information handling system 1000. A hard disk drive (HDD)
controller system 1028 may couple one or more hard disk drives or
the like to information handling system, for example Serial
Advanced Technology Attachment (Serial ATA) type drives or the
like, or alternatively a semiconductor based drive comprising flash
memory, phase change, and/or chalcogenide type memory or the like.
Switch 1030 may be utilized to couple one or more switched devices
to I/O bridge 1022, for example Gigabit Ethernet type devices or
the like. Furthermore, as shown in FIG. 10, information handling
system 1000 may include a baseband and radio-frequency (RF) block
1032 comprising a base band processor and/or RF circuits and
devices for wireless communication with other wireless
communication devices and/or via wireless networks via antenna
1034, although the scope of the claimed subject matter is not
limited in these respects.
[0037] In one or more embodiments, information handling system 1000
may include MEMS projector module 1036 that may correspond to MEMS
platform 114 of FIG. 1, and which may include any one or more or
all of the components of scanned beam display 900 of FIG. 9, for
example controller 122, horizontal drive circuit 918, vertical
drive circuit 920, and/or laser source 910. In one or more
embodiments, projector 1036 may be controlled by one or more of
processors 1010 and/or 1012 to implements some or all of the
functions of controller 122 of FIG. 1. In one or more embodiments,
projector 1036 may comprise a MEMS based scanned beam display for
displaying an image projected by projector 1036 where the image may
likewise be represented by target/display 1040. In one or more
embodiments, a scanned beam projector may comprise video/graphics
block 1020 having a video controller to provide video information
1038 to projector 1036 to display an image represented by
target/display 1040. In one or more embodiments, MEMS module 1036
may comprise one or more components of scanned beam display 900 of
FIG. 9, for example MEMS platform 114 having one or more PZR
sensors 210 as shown and described herein. However, these are
merely example implementations for projector 1036 within
information handling system 1000, and the scope of the claimed
subject matter is not limited in these respects.
[0038] Although the claimed subject matter has been described with
a certain degree of particularity, it should be recognized that
elements thereof may be altered by persons skilled in the art
without departing from the spirit and/or scope of claimed subject
matter. It is believed that the subject matter pertaining to
piezoresistive sensors for a MEMS device having rejection of
undesired motion and/or many of its attendant utilities will be
understood by the forgoing description, and it will be apparent
that various changes may be made in the form, construction and/or
arrangement of the components thereof without departing from the
scope and/or spirit of the claimed subject matter or without
sacrificing all of its material advantages, the form herein before
described being merely an explanatory embodiment thereof, and/or
further without providing substantial change thereto. It is the
intention of the claims to encompass and/or include such
changes.
* * * * *